Voice coil motor (VCM) is widely used in the electronic products. In recent years it is a popular solution for driving and focusing the lenses of cameras of smart phones and tablet computers. By taking signals from an image signal processor (ISP), a driver IC produces the corresponding current output to drive a voice coil actuator, thus changes the lens position to reach the focus point. Since voice coil actuator changes lens position linearly for a short distance, it is widely used for precise tracking or focusing. VCM actuator can be modeled as a second order under damped system. As the size of VCM is getting smaller and the weight of VCM is getting lighter, the system damping ratio of the VCM actuator becomes smaller. When a driving current applied to the coil of VCM, the Lorentz force generated by electromagnetic force of the coil in a magnetic field from a permanent magnet moves lens linearly. However, the VCM actuator as a second order under damped system will cause mechanical oscillation. The lens will oscillate around the final focus position for a while, leading to a long focus time for the lens. The smaller the damping ratio is, the longer the time the lens settles to its final position.
In order to develop cameras with fast focus time, several methods in the art of driving VCM actuator have been suggested and practiced to eliminate the oscillation or decrease the oscillation magnitude. For example, one can increase the system damping ratio to limit the oscillation, so as to achieve fast focus. However, increasing the system damping ratio will increase the power consumption of the driving system, decrease the battery time, and therefore it is not practical and is not adopted in a camera of a mobile system.
US Patent Application No. 20110156763 discloses a two-step driving method to achieve a fast focusing speed. This method relies on a precise knowledge about the natural frequency of each individual VCM in order to provide the amplitudes for the two step signals to form a fast system response. In practice, every coil may have a slightly different natural frequency from the targeted manufacturer's design value, deviating from an actual natural frequency may lead to residual oscillation after the lens reaches the desired position, and therefore increase actual focusing time. Furthermore, in order to implement this method without a complicated driver device design, an assumption that the damping coefficient is zero has to be made. This assumption may further depart away from an actual VCM behavior, resulting in additional factors causing the residual oscillation.
In view of the above problems of controlling mechanical oscillations in an under damped second order VCM system, there are needs to improve the settling speed of a VCM, minimize the residual oscillation, and simplify the driver device design and configuration.